Most any application needs some form of persistence—a way to store the data outside of the application for safekeeping. The most basic way is to write data to the file system, but that can quickly become a slow and unwieldy way to solve the problem. A full-blown database provides a powerful way to index and retrieve data, but may also be overkill. Sometimes all you need is a quick way to take a freeform piece of information, associate it with a label, stash it somewhere, and pull it back out again in a jiffy.

Enter the key-value store. It’s essentially a database, but one with a highly specific purpose and a deliberately constrained design. Its job is to let you take data (a “value”), apply a label to it (a “key”), and store it either in-memory or in some storage system that’s optimized for fast retrieval. Applications use key-value databases for everything from caching objects to sharing commonly used data among application nodes.

Many relational databases can function as key-value stores, but that’s a little like using a tractor-trailer to go on grocery runs. It works, but it’s dramatically inefficient, and there are far less top-heavy ways to solve the problem. A key-value store provides just enough infrastructure for simple value storage and retrieval, integrates more directly with applications that use it, and scales in a more granular way with the application workload.

Here we’ve examined five widely used products (including one cloud service) that are explicitly billed as key-value databases, or which offer key-value storage as a central feature. All have their differences. Hazelcast and Memcached tend toward minimalism, and don’t even bother to back the data in question on disk. Aerospike, Cosmos DB, and Redis are fuller featured, but still revolve around the key-value metaphor.

See the table below to compare features. Read on for brief discussions of each database.